1. Field of the Invention
The present invention relates to electro-optical measuring instruments and to optical routeing modules and gating devices suitable for use therein.
2. Discussion of Prior Art
With ever-increasing miniaturization of electronic circuits there is a need for increasingly sophisticated analytical techniques operating at ever higher resolutions. One such technique involves the use of Photoluminescence Lifetime Spectrometers (PLS) for measurement of photoluminescence in semi-conductors especially those of compounds such as Gallium Arsenide (Ga As) which are more susceptible to the incidence of structural discontinuities due to local crystallisation defects, such defects being detectable by variations in the photoluminescent output thereat.
In more detail, photoluminescence is the emission of a photon upon recombination of an electron-hole pair generated by photoexcitation of a semiconductor. The term photoluminescence is normally used to describe this mechanism in solids, whereas the term fluorescence describes analogous processes in atoms and molecules.
The photoluminescence intensity is related to the number of excited electron-hole pairs, or, in other words, the excess carrier densities. These carriers eventually decay, the carrier lifetime being determined by the rates of decay via various (electron-hole pair) recombination mechanisms. Photoluminescence is the result of a radiative recombination process and can be observed externally, due to photons leaving the sample surface. The carrier lifetime can be determined from the photoluminescence lifetime by straightforward interpretation.
The carrier lifetime and the closely related carrier diffusion length are the most important parameters characterising the electronic properties of a semiconductor In order to predict and explain semiconductor device performance, these two parameters have to be known.
In gallium arsenide (GaAs), the technologically second most important semiconductor after silicon (Si), the carrier lifetime is of the order of 10 ps to 1 .mu.s (10-.sup.11 -10.sup.-6 s), depending on the influence (presence/absence) of localized non-radiative recombination centers. It is hence obvious that spatial fluctuations of the carrier lifetime can occur on a scale comparable to the carrier diffusion length, typically below 1 to 10 .mu.m. In fact, strong inhomogeneities in the carrier lifetime have been experimentally observed by averaging over about 100 .mu.m, i.e. many times the diffusion length in GaAs, but closer investigation has so far been impossible due to lack of an experimental technique.
It is known that integration density of gates on GaAs chips is still very low compared with Si, because of the inhomogeneity of GaAs wafers. hence there is great commercial interest in experimental methods capable of measuring the carrier lifetime of such materials with high temporal and high spatial resolution
Time-correlated single photon counting (TCSPC) is an experimental technique for measuring the dynamic behaviour of excited electronic states in atoms, molecules and solids. At present, this technique is widely applied in photochemistry and photobiology with many commercial systems for fluorescence decay measurements already on the market. However, due to lack of fast single photon detectors with high sensitivity in the near infrared, its application to semiconductors has so far been very limited.
It is important to realise, that the TCSPC technique is several orders of magnitude more sensitive than any other experimental technique for measuring time-resolved photoluminescence. This permits the measurement of photoluminescence with very high spatial resolution. With other techniques a large photoexcitation density is required in order to obtain a sufficiently large signal from a small sample area. However, the upper limit for the tolerable excitation density is often the doping concentration. In other cases, e.g. in testing a laser diode resonator structure of typically 50.times.3.times.1 .mu.m.sup.3 size, the excitation density may have to be even lower in order to stay below the onset of induced emission.
Taking into account only the inevitable losses occurring in the sample itself, internal quantum efficiency, geometric factors, surface reflection etc., the signal intensity available from a sample area of only a few .mu.m diameter can be as low as a few photons per excitation pulse. Even with a highly efficient spectrometer, this signal intensity is reduced even further by spectral discrimination etc. before being detected.
Previously known apparatus is unable to separate out and extract such very low intensity output signals at the high spatial resolutions required to pin-point any microscopic defects that may be present on the semi-conductor surface or in the body volume probed by the photoexcited carriers. In addition there is the major problem, once the occurrence of individual luminescence photons has been accurately detected, of measuring the elapsed time between the detected photons and the associated excitation pulses which originally gave rise to them, given the extremely large numbers of excitations pulses for which no output photons are detected, as well as the very high excitation pulse frequencies which are used in practice in these studies.